Difference between block size and cluster size

I've got a question concerning the block size and cluster size. Regarding to what I have read about that I assume the following: * The block size is the physical size of a block, mostly 512 bytes. There is no way to change this. * The cluster size is the minimal size of a block that is read and writable by the OS. If I create a new filesystem e.g. ext3, I can specify this minimal block size with the switch -b. Almost all programs like dumpe2fs, mke2fs use block size as an name for cluster size. If I have got the following output: $ stat test File: `test' Size: 13 Blocks: 4 IO Block: 2048 regular file Device: 700h/1792d Inode: 15 Links: 1 Is it correct that the size is the actual space in bytes, blocks are the physically used blocks (512 bytes for each) and IO block relates to the block size specified when creating the FS?